1. Field of the Invention
The invention relates in general to a method for reducing the electromagnetic irradiation of a monitor. In particular, the invention relates to a software method for reducing the electromagnetic irradiation of the Phase Lock Loop (PLL) within an On-Screen Display Menu system.
2. Description of the Related Art
FIG. 1 is a typical response characteristic of an electromagnetic irradiation. In FIG. 1, a characteristic waveform depending on the frequency w can be represented by the function of f (w)=Asin (nwt), wherein A is the amplitude of the function, w is the frequency of the function, and n is the integer harmonic. The function can be further expanded as follows:
f(w)=A1 sin (wt)+A2 sin(2wt)+A3 sin(3wt)+ . . . 
The above equation, as shown in FIG. 1, is a frequency response with the harmonics incurred by a typical electromagnetic irradiation. Commonly, for a PLL circuit with an OSD system, the shown frequency of the electromagnetic irradiation is determined as set the resolution of the PLL circuit during the circuit layout. Typically, the exceeding electromagnetic irradiation appears on the positions of frequencies (Horizontal Resolution X Horizontal Frequency X n (n=2, 3, 4 . . . ) when the measurement of an OSD system. Particularly, the highest electromagnetic irradiation is easily created at the positions of n=2, 3, 4 . . . as shown in FIG. 1.
A manner for reducing the exceeding electromagnetic irradiation problem is to redesign the circuit layout or to add capacitor or inductor. No matter to redesign the layout or to add the element of capacitor or inductor could be causing the circuitry complexity, even incurring the noise interference from the added element. Another manner is to close the PLL OSC of the OSD system. However, the OSD system is often used with other circuits, such as with a Pulse Width Modulation (PWM) circuit for controlling the output power. To keep a normal operation in the whole system, therefore, the PLL OSC cannot be closed.
An object of the invention is to provide a software solution method for reducing electromagnetic irradiation, which adjusts the display resolution in order to avoid specific frequencies with the excess electromagnetic irradiation, thereby reducing the exceeding electromagnetic irradiation from the PLL OSC within the OSD system, and so it is unnecessary to redesign the circuit layout or to add new element for reducing the irradiation. The method includes the following steps: setting the resolution of an OSD system to a predetermined pass value of Electromagnetic Interference (EMI); checking the OSD system; changing the resolution of the OSD system to a value to be displayed if the OSD system is started; keeping on the predetermined value of EMI if the OSD system is not started; repeating from the checking the OSD system step.
As mentioned above, the software method of the present invention can easily determine the current resolution value without redesigning the circuit layout or adding an extra element. Thus, the excess irradiation of the OSD system is reduced and the PLL is operated at the normal frequency to keep the high resolution of the OSD system.